In recent years, magnetic random access memories (hereinafter referred to as MRAMs) using the magnetoresistive effect of ferromagnetic materials have been drawing increasing attention as the next-generation solid-state nonvolatile memories that can cope with high-speed reading and writing, large capacities, and low-power-consumption operations. Particularly, there has been increasing interest in magnetoresistive elements having ferromagnetic tunnel junctions since the high rate of change in magnetoresistance of such magnetoresistive elements was discovered. A ferromagnetic tunnel junction has a three-layer stack structure formed by stacking a recording layer having a changeable magnetization direction, an insulator layer, and a fixed layer that is located on the opposite side from the recording layer and maintains a predetermined magnetization direction.
Magnetoresistive elements having such ferromagnetic tunnel junctions are also called MTJs (Magnetic Tunnel Junctions). As a write method to be used in such magnetoresistive elements, there has been suggested a write method (spin torque transfer switching technique) using spin momentum transfers (SMTs). According to this method, the magnetization direction of a recording layer is reversed by applying a spin-polarized current to the magnetoresistive element. Furthermore, as the volume of the magnetic layer forming the recording layer is smaller, the spin-polarized electrons to be injected can be fewer. Accordingly, this method is expected to be a write method that can achieve both device miniaturization and lower currents.
As the ferromagnetic materials forming such a magnetoresistive element, so-called perpendicular magnetization films each having an easy axis of magnetization in a direction perpendicular to the film plane have been considered. In a case where a magnetic crystalline anisotropy is used in a structure of a perpendicular magnetization type, shape anisotropies are not used, and accordingly, the device shape can be made smaller than that of an in-plane magnetization type. Also, variance in the easy axis of magnetization can be made smaller. Accordingly, by using a material having a large magnetic crystalline anisotropy, both miniaturization and lower currents can be expected to be achieved while a thermal disturbance resistance is maintained.
Where a MTJ is formed as a device of a perpendicular magnetization type, the materials of the recording layer and the base layer for adjusting the crystalline orientation are diffused due to a heat treatment, and the MR ratio becomes lower. To counter this problem, there has been a known technique for achieving a high MR ratio by forming a crystallization acceleration film that accelerates crystallization and is in contact with an interfacial magnetic film having an amorphous structure. As the crystallization acceleration film is formed, crystallization is accelerated from the tunnel barrier layer side, and the interfaces with the tunnel barrier layer and the interfacial magnetic film are matched to each other (JP-A 2009-081216 (KOKAI), for example). By using this technique, a high MR ratio can be achieved. However, with the heat treatment in the device manufacturing process being taken into consideration, the MR ratio and the resistance ratio preferably do not change when a heat treatment is further performed after the initial state.